The discussion centers around recent experiments testing Bell's theorem, specifically focusing on loophole-free tests involving entangled photons. Participants explore the implications of these tests for local realism and the assumptions underlying the experiments, as well as the significance of the results presented by different research teams.
Participants express a range of views, with some agreeing on the closure of certain loopholes while others challenge the definitions and implications of the experiments. The discussion remains unresolved regarding the strength of the claims and the interpretation of results.
There are limitations regarding the assumptions made in defining "loophole-free," and the dependence on specific criteria set by different research teams. The discussion reflects ongoing scrutiny and debate within the community about the implications of these experiments.
Demystifier said:I dispute 1). For instance, unitary evolution is compatible with Bohmian mechanics (BM), and BM is compatible with definite outcomes of measurements.
Bell only says that a value at a location of an object isn't enough to contribute exclusively to its state, there has to be non local value attributed, but does that mean one value interferes with all the universe's wavefunctions in all directions or can we isolate a chuck of space-time and call it all inclusive? Entanglement swapping seems to confirm that Bell's Inequality can be observed over mind-boggling distance of space and time. It contradicts our classic deterministic existence we have known since the see-saw.stevendaryl said:So even though a "spin measurement" has a definite outcome, it can't be interpreted--as it is in mainstream QM--as the particle being a state of definite spin.
Not quite. It is my understanding that in BM there is still some overlap, however small, of different components of the wave functions related to different values of position. I guess we discussed this question before: https://www.physicsforums.com/threa...nt-or-not-to-standard-qm.307641/#post-2167542 , https://www.physicsforums.com/threa...-entanglement-and.369328/page-15#post-2602406 , https://www.physicsforums.com/threa...-entanglement-and.369328/page-16#post-2603650 .Demystifier said:I dispute 1). For instance, unitary evolution is compatible with Bohmian mechanics (BM), and BM is compatible with definite outcomes of measurements.
I don't understand why this is necessarily many-worlds. It is not obvious that this superposition cannot exist in one and only world - ours. If you believe this is impossible, and, say, you cannot be in Hawaii and in Paris at the same time in principle, then maybe you don't think unitary evolution is universal and maybe you don't think the Schroedinger's cat can exist in principle. You can take such a position, but that does not mean I have to take such a position.stevendaryl said:Well, it seems to me that it is. Suppose that I prepare an electron to have spin-up in the z-direction. Then I plan to measure the electron's spin in the x-direction. I resolve ahead of time: If it's spin-up, I will go to Paris for vacation. If it's spin-down, I will go to Hawaii. Then, after I've done my measurement, and gone on my vacation, it seems that someone from a distant planet describing the whole Earth using quantum mechanics would have to describe it as a superposition of an Earth where I am in Hawaii and an Earth where I am in Paris. That's many-worlds, it seems to me.
I believe this derivation is indeed necessary. Let me remind you that the Bell theorem is 50+ years old, whereas there had been no loophole-free experimental demonstrations of violations of the inequalities at least until Q3 2015:-). And it is not obvious that the recent experiments offer such demonstration, e.g., for the reason we are discussing now. So without this derivation one could say: these inequalities must be satisfied in local realistic theories? So what? That does not contradict any experiments. At least it had not contradicted until recently.The strong and necessary point of the Bell theorem was that it demonstrated incompatibility of local realistic theories with predictions of quantum theory.stevendaryl said:Yes, but in a sense, that derivation is unnecessary, since experiments prove the violation. This "second part" that you're talking about is just a calculation that shows that the experimental results are in fact predicted by QM (or at least, is predicted by a particular "recipe" for using QM).
Yes, there is still some overlap between wave functions. But definite outcomes occur in the sense that particles attain definite positions. Even if the overlap was large, definite outcomes would still occur in that sense.akhmeteli said:Not quite. It is my understanding that in BM there is still some overlap, however small, of different components of the wave functions related to different values of position. I guess we discussed this question before: https://www.physicsforums.com/threa...nt-or-not-to-standard-qm.307641/#post-2167542 , https://www.physicsforums.com/threa...-entanglement-and.369328/page-15#post-2602406 , https://www.physicsforums.com/threa...-entanglement-and.369328/page-16#post-2603650 .
akhmeteli said:I believe this derivation is indeed necessary. Let me remind you that the Bell theorem is 50+ years old, whereas there had been no loophole-free experimental demonstrations of violations of the inequalities at least until Q3 2015:-).
And it is not obvious that the recent experiments offer such demonstration, e.g., for the reason we are discussing now. So without this derivation one could say: these inequalities must be satisfied in local realistic theories? So what?
akhmeteli said:I don't understand why this is necessarily many-worlds.
It is not obvious that this superposition cannot exist in one and only world - ours.
There was another thread on the Delft experiment, but it has now been closed. You can discuss all experiments here.nikman said:There are three all-loopholes-closed experiments now: Delft, Vienna and NIST. Is this the thread for discussion of all of them or is there another thread I haven't sussed out?
DrClaude said:There was another thread on the Delft experiment, but it has now been closed. You can discuss all experiments here.
OK, so I'm having hard time trying to understand if you agree or disagree with 1) :-) "Philosophically, the two are incompatible", but "there is no inconsistency between assuming unitary evolution (now) and definite outcomes (at some point)" :-) But I conclude that, at least "philosophically" the assumption of fixed outcomes used in the experiments is incompatible with unitary evolution:-)stevendaryl said:Philosophically, the two are incompatible, but strangely don't seem to lead to any inconsistencies in practice.
For practical purposes, the Von Neumann recipe seems to work very well:
The second step appears to violate unitary evolution. However, I think most people believe that you could, in principle, if not in practice, consider the evolution of "System + Measuring Device" to be unitary, as well, and push off the "collapse" until a later time. If you can always push off the collapse until later (and you can, if you're willing to consider the quantum mechanics of arbitrarily large systems), then there is no inconsistency between assuming unitary evolution (now) and definite outcomes (at some point).
- Between observations, the wave function evolves unitarily.
- After a measurement, the wave function collapses into the eigenstate of the observable corresponding to the measured eigenvalue.
Well, I used to think that "The Many-Worlds Interpretation (MWI) of quantum mechanics holds that there are many worlds which exist in parallel at the same space and time as our own." (http://plato.stanford.edu/entries/qm-manyworlds/#2 ). But since you're telling me that ""many" in the many worlds is just picturesque language", I am at a loss:-)stevendaryl said:What do you think "many-worlds" refers to?
Well, the "many" in the many worlds is just picturesque language. Many-worlds is really about superpositions of macroscopically different states of the (one) universe.
I agree that it is better to be rich and healthy than sick and poor:-) But we only have the results of those experiments that we have now, not a hundred years after our death:-)stevendaryl said:But the way to fix that is to have better experiments.
I am afraid experiments do not give us a definite answer so far. Indeed, they only demonstrate violations under the assumption of fixed outcomes. So we don't know if there are violations or there are just no fixed outcomes. Based on my belief in unitary evolution, I bet that there are no fixed outcomes and there are no violations. Your bet may be different.stevendaryl said:I have a completely different attitude toward the same facts. The interesting question is whether real experiments violate Bell's inequalities. The answer to that is to be found in experiments, not in derivations.
I don't understand how the particles attain definite positions, but maybe it's just my problem:-). However, following the mentor's heads-up:-), let us discuss this in the context of the recent experiments. You told me some time ago that the projection postulate is, strictly speaking, approximate in the Bohmian interpretation. That means that if a measurement has given us some value of the position of the particle, we cannot be sure that an immediate repeat measurement will give the same result (by the way, this is the reason why I cannot understand how the particles obtain definite positions). That means that outcomes are not really fixed. Therefore, the interpretation of the recent experiments is based on an assumption that, strictly speaking, is not correct in the Bohmian interpretation.Demystifier said:Yes, there is still some overlap between wave functions. But definite outcomes occur in the sense that particles attain definite positions. Even if the overlap was large, definite outcomes would still occur in that sense.
akhmeteli said:So we don't know if there are violations or there are just no fixed outcomes. Based on my belief in unitary evolution, I bet that there are no fixed outcomes and there are no violations. Your bet may be different.
First, any interpretation of any experiment in nature is based on an assumption that, strictly speaking, is not correct. Physics (unlike mathematics) is a FAPP science.akhmeteli said:I don't understand how the particles attain definite positions, but maybe it's just my problem:-). However, following the mentor's heads-up:-), let us discuss this in the context of the recent experiments. You told me some time ago that the projection postulate is, strictly speaking, approximate in the Bohmian interpretation. That means that if a measurement has given us some value of the position of the particle, we cannot be sure that an immediate repeat measurement will give the same result (by the way, this is the reason why I cannot understand how the particles obtain definite positions). That means that outcomes are not really fixed. Therefore, the interpretation of the recent experiments is based on an assumption that, strictly speaking, is not correct in the Bohmian interpretation.
akhmeteli said:Well, I used to think that "The Many-Worlds Interpretation (MWI) of quantum mechanics holds that there are many worlds which exist in parallel at the same space and time as our own."
I am hardly a subscriber to MWI, but if we are considering any loophole what-so-ever, MWI does introduce a somewhat bizarre (at least in human terms) loophole to the Bell experiments. If we assume that once Bell-compliant measurements are made that the Bell-compliant result can cause likely doom to the measurement results, then our ability to examine the results as posted in arxiv is evidence that we are in a select MWI world where we lucked out because of the atypical, non Bell-supportive results.gill1109 said:If you believe in the many world theory then (it seems to me) you do not believe in any ordinary reality at all. There are no "measurement outcomes". Not ever. They are somehow illusory. Many world theorists do not have any problem with non-locality because there is no classical world where measurements actually have outcomes.
Demystifier said:First, any interpretation of any experiment in nature is based on an assumption that, strictly speaking, is not correct. Physics (unlike mathematics) is a FAPP science.
You probably have in mind the picture of Bohmian mechanics where there is a wave function, on the one hand, and a point particle somewhere there, on the other hand. This may be a correct picture of reality, or an incorrect picture, I don't know. However, I am not sure we can determine the position of that point particle, when we conduct a measurement, we just have unitary evolution in Bohmian mechanics, nothing more, as far as I can see, and unitary evolution cannot definitely determine the true position of the point particle, as far as I understand. Furthermore, let us leave alone definite outcomes for a moment. If you agree that the outcomes are not fixed, then the assumption used for interpretation of the new experiments is, strictly speaking incorrect.Demystifier said:Second, just because outcomes are not fixed does not mean that they are not definite. For instance, the shape of a flag on wind is definite but not fixed. This means that the shape changes with time (so is not fixed), but at each given time the flag has some definite shape.
I don't see how macroscopic superpositions imply many-worlds. I can (and want to) take the following position: macroscopic superpositions, if unitary evolution predicts them, exist just in one world, ours. A superposition of two different US presidents or the Schroedinger's cat may seem crazy, but I don't know how you can bury such a position without burying unitary evolution in the same grave.stevendaryl said:Yes, in that sense, macroscopic superpositions imply many-worlds. The many worlds in MWI are different states of the same universe. So if there is a possible state of the world in which (say) Al Gore won the election in 2000, and a state of the world in which George Bush won, then you can form a superposition of the two states. MWI is about such macroscopic superpositions. The "many" refers to many different macroscopically distinguishable states that exist at the same time as elements of a superposition.
It is natural to believe that measurement in quantum mechanics can be described as unitary evolution of the larger system including the initial system under measurement and the instrument. It is well-known that unitary evolution cannot, say, turn a pure state into a mixture, that means that, strictly speaking, we can never (or hardly ever) have fixed outcomes of measurement. This is one of the aspects of the measurement problem of quantum theory. So how is this related to the recent experiments on the Bell inequalities violation? The interpretation of all of these experiments assumes fixed outcomes at some point. However, unitary evolution of quantum theory does not allow such fixed outcomes. Therefore, it looks like the conclusion of the experiments is: local realistic theories are wrong, assuming that quantum theory is wrong. So we cannot bury local realistic theories using the results of these experiments without burying quantum theory in the process. This seems a little bit confusing:-)morrobay said:Could you elaborate/explain " no fixed outcomes " Is this related to non realism or non deterministic hidden variables
or related to detector /particle interactions in context of inequality violations ?
akhmeteli said:The interpretation of all of these experiments assumes fixed outcomes at some point. However, unitary evolution of quantum theory does not allow such fixed outcomes. Therefore, it looks like the conclusion of the experiments is: local realistic theories are wrong, assuming that quantum theory is wrong. So we cannot bury local realistic theories using the results of these experiments without burying quantum theory in the process.
First of all, according to M. Schlosshauer (Annals of Physics, 321 (2006) 112-149), "no positive experimental evidence exists for physical state-vector collapse". The same source states that "the universal validity of unitary dynamics and the superposition principle has been confirmed far into the mesoscopic and macroscopic realm in all experiments conducted thus far". Third, the von Neumann's picture that you use (unitary evolution between measurements, collapse during measurements) seems a bit strange: if you call something "a measurement", you get one result, if you don't call the same process "a measurement", you get a different result (using unitary evolution of the larger system including the instrument). Of course, you can say that this is actually perfectly OK, and there is no measurement problem in quantum mechanics, but not everybody agrees. And let me repeat this: there is no experimental evidence of failure of unitary evolution.nikman said:How is it necessary to assume wrongness of quantum theory in connection with the experiments? The measurement "collapses the wavefunction" rendering it no longer unitary and yields an eigenvalue. The eigenvalue is a classical artifact. So, in effect all measurements are classical. We're exploring the quantum domain using classical concepts and classical tools and the results we obtain are classical, not quantum, results.
I agree, no local realist theories can replicate the predictions of QM, if you include collapse in QM. But there is no evidence of collapse, so, as of now, local realistic theories do not need to emulate collapse to be in agreement with experiments.nikman said:As Wheeler once put it, what we "know" is analogous to a landscape of tops of fence-posts sticking up out of snowdrifts. BT doesn't say that nonlocality and/or nonrealism is or are objective truths -- it says that no model which assumes local realism can replicate the predictions of QM. The results of the Bell experiments demonstrate that EPR can't prove their contention that quantum theory is incomplete. They set the rules; they lose the game.
akhmeteli said:And let me repeat this: there is no experimental evidence of failure of unitary evolution.
Maybe measurement results in something important, maybe it does not, but there is no experimental evidence of a failure of unitary evolution. What you say about this bit measurement is not a proof of the opposite. The particle has interacted with the instrument, no wonder that it is much more difficult to observe coherence in the enlarged system including the instrument. The same is true about decoherence. If you cannot find coherence, that does not mean it disappeared. Each time you can fully control your system, even if it consists of hundreds of particles, you see coherence. If you cannot control your system, you cannot blame unitary evolution. There is no evidence of failure of unitary evolution, so why say it fails? Again, you may believe that it fails, but I am under no obligation to believe that until experiments show such failure.nikman said:Don't know what this means. I put "collapses the wavefunction" in scare quotes for a reason I assumed was obvious. Bohr for example considered state vector collapse a mathematical convenience possessing no physical reality. Dieter Zeh and the Many Minds school disown collapse as conventionally understood; there's seamless evolution into decoherence with a substrate of coherence apparently preserved. But everyone agrees that measurement results in something important happening. Ordinarily it relates to decoherence in one form or another, or else incoherence. In quantum information theory a single particle carries one bit of coherent classical information. You measure the particle and extract that bit and then measure the particle again and find you receive incorrect data if not irreducible randomness which some would simply call gibberish. But from whatever direction you approach it, measurement is life-changing. Unitary evolution is disturbed or disrupted or destroyed; so why not say it fails.